Wireless power transfer (WPT) based on resonant inductive coupling is a process of energy transfer between two or even more physically-unconnected objects through electromagnetic induction. Transferring electric power without using power cables is more convenient and safer in some applications. An efficient WPT system can greatly promote advances in portable electronic devices, micro robotics and medical equipment, whose functions are often limited by a need for power. Furthermore, the availability of high-power WPT systems can promote popularity of electric vehicles (EVs) significantly.
The technology of WPT through resonant inductive coupling was first introduced by the MIT in 2007 [1], where self-resonant coils in a strongly coupled regime were used to achieve WPT. Resonant-coupling WPT uses two copper coils tuned to resonate at the same resonant frequency. The coupled-mode theory (CMT) was applied to decouple the resonating coils and then analyze the WPT system.
Since then, the technology of WPT has gone through dynamic evolutions in recent years. In most cases, two-coil systems (each with one transmitter coil and one receiver coil) are used [2]-[4]. The use of lumped equivalent circuits replaces the CMT in analyzing these systems. To raise the delivered power and to improve the transfer efficiency, considerable research has been carried out regarding these systems. Various techniques, including impedance matching, frequency tuning, coupling efficiency improvement and others [5]-[8], have been considered. Finite element analysis (FEA) is usually used for magnetic simulation and coil design. Multi-coil systems with multiple transmitters or multiple receivers have also been introduced [9], [10]. These multi-coil systems are good solutions for wirelessly powering several devices at the same time. In general, a multi-coil system can be equivalently viewed as consisting of a number of two-coil subsystems with mutual coupling among the coils in the multi-coil system.
The two-coil topology works quite well in low-power applications (defined by the Wireless Power Consortium, WPC, as applications with a wireless transfer within 0 W to 5 W). When used in high-power applications such as recharging batteries for EVs, however, the resonant current on the transmitter's resonant tank becomes undesirably high. This undesirably high resonant current requires a high terminal voltage across the resonant tank to sustain. The need for the high terminal voltage requires a power source that provides the terminal voltage to be able to supply a very high output voltage. The need to supply the very high output voltage leads to voltage stress at the power source. The occurrence of voltage stress is particularly undesirable for a WPT system in that a high operating frequency (e.g., 9.9 MHz [1]) is usually used for the power source to excite the transmitter's resonant tank. It is difficult and costly to implement the power source that operates at high frequency as well as needs to provide the very high output voltage.
There is a need in the art for a WPT technique that discourages occurrence of voltage stress at the power source. Such WPT technique is useful not only for high- or medium-power applications but also for low-power ones. The use of a power source requiring an output of only a low voltage may help development of battery-supported WPT systems, for instance. In one scenario, a battery-supported transmitter may be used to urgently recharge a miniaturized battery in a medical implant, such as a cardiac pacemaker, for emergency life saving.